The embodiments described herein relate generally to methods and systems that facilitate reducing noise in reinforced skin structures.
At least some known reinforced skin structures generate noise and vibrations. For example, reinforced structures such as aircraft, trains, automobiles, ships and the like may be subject to self-generated vibration due to a running powerplant, or other noise such as wind and ground noise. For aircraft in particular, noise and vibrations may be generated by the engines, wind effects from the aircraft's aerodynamics, flight loading, and/or other components of the aircraft. The noise and vibrations can propagate about the passenger cabin, such that passenger communication and comfort can be adversely effected. As a result, it is desirable to maintain interior cabin noise levels as low as possible. When weight and size are insignificant factors, wide latitude may be afforded in methods for reducing noise. However, in some environments, such as in aircraft, high power and light weight requirements impart restrictions on the methods one may use to reduce noise and vibration, in comparison to other forms of transportation vehicles and structures.
As described in U.S. Pat. No. 4,635,882, an aircraft structure may be designed such that the fundamental frequency of the skin is higher than the fundamental frequency of the stringers when the aircraft is unpressurized if it is desired to reduce low to mid frequency interior noise during takeoff and during cruise. Such result is achieved by controlling the relationship between stringer spacing and skin thickness so that such frequency relationship exists. However, when the skin fundamental frequency is higher than the fundamental frequency of the stringers, the coupled mode of the structure is a strong radiator of sound because a large section of the skin vibrates in phase. The response of the coupled mode on low to mid frequencies is strongly determined by the deflection of the stringers. Thus, damping the stringers is an effective way of reducing the vibration and noise.
Aircraft cabin noise above 600 Hz may be reduced with skin damping and insulation. However, such noise-reducing methods are generally ineffective for mid-frequency ranges of about 200-600 Hz, which generally occur when pressurized aircraft fuselages are flown at high altitudes. Such mid-frequency ranges may overlap with the resonant frequency of the aircraft fuselage's support members, or stringers, and excite a first bending and/or torsional twisting mode of the stringers, which may undesirably increase or amplify noise and vibrations in the fuselage.
One known method of reducing noise and vibration in an aircraft cabin includes attaching lead blankets to stringers and skin of the aircraft. The lead blankets are effective at absorbing vibrations; however, the lead blankets add substantial amounts of weight, causing inefficient operation of the aircraft, such as increased amounts of fuel consumed.
Another known method of reducing vibration and noise in the mid-frequency ranges in the cabin of an aircraft includes attaching stiffeners and damping materials to the stringers of the fuselage. Known stiffeners are generally either ineffective at reducing noise and vibrations or inefficient for aircraft operation. For example, one known stiffener is fabricated as a solid piece of material that is coupled directly to the flanges of a hat cross-sectional stringer, such that the space between the opposing flanges, defining a U-shape of the stringer, is completely covered. This known stiffener is sometimes referred to as a “full-hat” damper because it fully covers the hat-section of the stringer. However, the full-hat damper is undesirable because it prohibits visual corrosion inspection of the covered portions of the stringer. As such, the full-hat damper is typically applied to a stringer intermittently along certain sections of the stringers, leaving gaps therebetween to enable corrosion inspections to be performed. However, coupling the full-hat damper intermittently along the stringers reduces the effectiveness in constraining the bending and torsional twisting modes of the stringers. Moreover, because the full hat dampers are a solid piece of material, the full hat dampers add undesirable additional weight to the fuselage, resulting in decreased efficiency.
Other known stiffener configurations, such as ladder-shaped stiffeners/dampers that are attachable to a stringer, include square cutouts in the stiffener, such as those described in described in U.S. Pat. No. 4,635,882. However, such ladder-shaped stiffeners that include rectangular cutouts generally provide inadequate torsional twisting stiffness compared to full-hat stiffeners, which can result in resonant coupling of the stringer to the fuselage skin resulting in a failure to effectively transfer vibrational energy to the damping materials. Moreover, the effectiveness of most aircraft stiffeners with cutouts may be limited by structural limitations on the systems. For example, the power and weight requirements of modern aircraft may limit the type of damping material and/or the number of stiffening members that can be installed, which can result in reduced damping effect of these stiffening members.